A sodium-sulfur battery (hereinafter referred to as the NaS battery) is a high temperature secondary battery containing metal sodium and sulfur as active materials in an isolated manner using a solid electrolyte tube. When the NaS battery is heated at high temperature of about 300° C. a certain amount of energy is produced by an electrochemical reaction of both of the melted active materials. Normally, the NaS battery is formed by assembling a plurality of battery cells upright, and used in a form of a battery module including a plurality of battery cells connected together. That is, the battery module is a battery assembly where circuits (strings) each formed by connecting a plurality of battery cells in series are connected in parallel to form a block, and at least two blocks are connected in series.
The battery assembly heat insulating container is intended to maintain the high temperature state of the battery module operated at high temperature such as the Nas battery, and used for heat insulation between the internal space containing the battery module and the external space. In consideration of ease of assembling or handling, in general, the battery assembly heat insulating container adopts box shape structure including a box body having an opening on its upper surface and a lid body placed on the upper surface of the box body. The box body contains a battery module operated at high temperature (for example, see Japanese Patent No. 3693983).
Further, in order to realize such operation suitably, in the NaS battery, the battery assembly control device includes a module control device provided independently for each battery module and general-purpose equipment such as a sequencer. Each module control device measures the operating voltage and the operating temperature of each battery module, monitors the operating state, and regulates the operating temperature of the NaS battery by turning on/off a heater provided for each battery module. Further, by the current measurement function of the sequencer, for example, the electric discharging current of the NaS battery is measured to calculate the voltage drop in the sequencer, and discharge cut off voltage is determined so as to detect the end of electric discharging (depth of discharge) of the NaS battery (for example, see Japanese Laid-Open Patent Publication No. 2003-288950). The cut off voltage herein means voltage used as a reference in determining the end of electric charging or the end of electric discharging of the NaS battery.
Further, as a method of detecting a failure of the battery module (e.g., failure in each block), a method of detecting a failure of a battery module by comparing the depth of discharge in each block is disclosed (e.g., see Japanese Laid-Open Patent Publication No. 03-158781). In this method, the presence of the failure in the battery is considered for each of the blocks of the battery module. Therefore, in comparison with the method of detecting a failure in each of NaS battery cells forming the block, the apparatus is not complicated, and the production cost is reduced. In these respects, this failure detection method is advantageous.